The invention relates to systems for controlling material handling apparatus, and in particular to a system for sensing the positions of fork load wheels of a low lift pallet-type fork truck with respect to an opening in a pallet bottom deck, so that upon lowering the wheels to lift the pallet they and their linkages pass freely through the opening and do not engage and damage the bottom deck.
Various materials, such as food products and the like, are often carried on pallets for convenient transport by a fork lift truck from one location to another. Where the pallets are not required to be lifted to significant heights, the fork lift truck may be of the short lift pallet-type that lifts the pallets to only about 6" above the floor. Such fork lifts usually comprise a rearward operator cab and a pair of horizontal forks extending forwardly from the cab. Each fork carriers one or two load wheels connected to its bottom by a linkage, and the wheels are movable between retracted positions where the forks are lowered and extended positions where the wheels elevated and support forward ends of the forks.
Pallets transported by short lift fork trucks are usually of the 4-way entrance type. They are made of wood, have a top deck formed by a plurality of boards attached at their ends to wooden beams extending perpendicularly of the boards, and a bottom deck formed by a plurality of boards attached at their ends to the beams and defining two 9" wide openings extending perpendicularly of the beams and parallel to the boards about 27" apart from center-to-center. To provide rigidity for the center of the pallet, a center beam extends between and is attached to the top and bottom deck boards about midway between and parallel to the side beams. The three beams are relieved at each of the two openings to define channels through the pallet along the lengths of the openings. The forks of a low lift pallet-type truck are too wide to be extended lengthwise into and through the channels to lift the pallet, so to lift the pallet the forks are extended into passages defined between the top and bottom decks on opposite sides of the center beam, in a direction perpendicular to the bottom deck openings. The forks should be extended into the passages by an amount that positions the load wheels in alignment with the first or second opening, so that when the wheels are lowered to lift the pallet, they and their linkages pass freely through the opening without encountering and breaking any bottom deck boards. Lift trucks having a single wheel per fork require about 7" of opening width for free movement of the wheels and linkages through the opening, so with a 9" wide opening, 2" of leeway are available. If there are two wheels per fork, then about 8.75" of opening width is required for unobstructed movement of the wheels and their linkages, leaving only 0.25" of leeway.
When an operator extends the forks of a low lift pallet-type truck into the pallet in a direction perpendicular to the lengths of the bottom deck openings, because of the top deck and any load carried on the pallet he cannot readily visually observe the positions of the wheels with respect to the openings. If as a result of improper staging the wheels are not correctly positioned, upon lowering them to lift the pallet they and/or their linkages will engage and break one or more bottom deck boards. In addition to errors in staging, also attributing to pallet breakage are different pallet sizes and pallets not manufactured to specification.
Short lift fork trucks have a vertically extending plate at the rearward end of the forks, and one conventional practice contemplates that the wheels be located on the forks such that, upon extension of the forks into the pallet in a direction perpendicular to the lengths of the openings, when the rearward end of the pallet engages the plate, the wheels and their linkages will be positioned for unobstructed downward passage through the bottom deck openings. However, should a load overhang the rearward end of the pallet, the vertical plate will engage it instead of the pallet, the wheels will not be properly aligned with respect to the openings, and upon being lifted the pallet will be broken.